1. Field of the Invention
The present invention relates to an ion source of an electronic impact type for producing plasma by ionizing a gas by electronic impact in a magnetic field. More particularly, the present invention relates to an ion source which can increase the rate of multiply charged ion (ion of doubly charged or more) contained in an ion beam to be extracted.
2. Description of the Related Art
There are various systems of the ion sources of the electronic impact type. One of the examples is disclosed in Patent Laid Open 35648/1997, where an ion source of a Bernas-type is described for increasing the density of plasma by using in combination of confinement of electron by a magnetic field and reflection of electron by a reflector.
It has been demanded to extract the multiply charged ion, that is, ion of doubly charged or more, from the ion source for utilizing the same. This is because, in comparison with singly charged ion, the multiply charged ion enables to obtain an accelerating energy of times of the charged number (for example, in a case of doubly charged ion, two times,) at the same acceleration voltage, and thus the multiply charged ion may be easily converted to a high energy. In order to produce much multiply charged ion in this type of ion source, it is usually necessary to increase an average electronic energy in plasma. Therefore, the following measures have been attempted: (a) a magnetic field for confining electron is intensified, (b) a density of plasma is increased, or (c) an energy of primary electron from the electron producing source is increased.
The electron in plasma is composed of a primary electron (the energy is normally about tens of eV to hundreds of eV) from the electron producing source and a secondary electron (the energy is normally about several eV to tens of eV) released at the time of ionization of the primary electron which is in collision with a neutral gas. An electron (third electron and the following electrons) released at the time of collision of the secondary electron with the neutral gas is called as secondary electron inclusively in the specification.
Since the electron of high energy is needed to produce multiply charged ion (for example, more than tens of eV are needed for producing doubly charged ion), the secondary electron is scarcely contributive to produce multiply charged ion. The multiply charged ion is almost produced by the primary electron. In contrast, for producing a singly charged ion, the electron energy as high as the case of the multiply charged ion is not required, and so the secondary electron is much contributive to produce singly charged ion.
However, each of the measures shown in (a) to (c) allows much of the secondary electron as well as the primary electron to be produced. That is, in case multiply charged ion is much produced, the singly charged ion is produced much as well. Therefore, the rate of multiply charged ion contained in the ion beam to be extracted from the ion source is hardly increased.
Therefore, in order to increase the quantity of multiply charged ion beam, the whole ion beam current is inevitably increased. However, in case the whole ion beam current is increased so much, an electrode system for extracting the ion beam will cause troubles including beam current limitation owing to a space charge effect or occurrence such as discharge between electrodes. Further, although electric current applied to the power source for supplying an extraction voltage to the extraction electrode system becomes large, it is difficult to supply a large electric current in view of capacity of the extraction power source. Therefore, a limitation is present to increase the whole beam current, and it is difficult to increase the quantity of the multiply charged ion taking such measures.
It is an object of the invention to provide an ion source which can increase the rate of the multiply charged ion contained in plasma, and also in the ion beam, thereby to increase the quantity of the multiply charged ion to be extracted.
In order to accomplish the object above, the following means are adopted. According to the present invention, there is provided an ion source comprising:
a plasma production chamber having a gas introduction portion for introducing a gas into the plasma production chamber, and an ion extraction opening for extracting ion beam thereat;
an electron producing source for supplying electron into the plasma production chamber to ionize the gas by electronic collision, thereby to produce plasma;
a magnetic field generator for producing a magnetic field for confining the electron produced at the electron producing source within the plasma production chamber;
a positive electrode provided in the plasma production chamber as electrically isolated therefrom, and having three openings formed at least at both sides in a direction along the magnetic field and at a side of the ion extraction opening; and
a direct current bias power source for applying bias voltage to the positive electrode, the bias voltage being positive against the plasma production chamber.
Main working effects obtained by providing the positive electrode and the bias power source are following (1) and (2).
(1) Ion Pushing-Back Action by the Positive Electrode
The ion in plasma produced in the plasma production chamber is pushed back toward the plasma, because the ion in plasma has the same polarity as the positive electrode, by the positive bias voltage applied to the positive electrode, in the wall surfaces other than the opening of the positive electrode. The pushed back ion is subject to collision by the primary electron produced mainly in the electron producing source, so that the charged number is increased. Generally as to the rate of the ion producing possibility of n charged (nxe2x89xa72) ion, compared to (a) the possibility to produce the n charged ion from a neutral gas, (b) the possibility to produce the n charged ion from an nxe2x88x921 charged ion is by far large. According to the ion source, since the process of (b) may be efficiently utilized by use of the pushed-back ion (namely, what is already ionized), the multiply charged ion may be efficiently produced.
(2) Absorption of the Secondary Electron by the Positive Electrode
The primary electron produced in the electron producing source is trapped by a magnetic field produced by the magnetic field generator and is moved following the magnetic field. In the moving process, the primary electron comes in collision with a neutral gas to produce the plasma. Since the primary electron has a comparatively high energy as above described, this contributes to production of the singly charged ion and the multiply charged ion.
In the neighborhood of the thus produced plasma, there is the positive electrode to be applied with the positive bias voltage from the bias power source. The secondary electron released at the time of collision of the primary electron with the neutral gas has the comparatively low energy as above mentioned and is released indefinitely in many directions. Thus, owing to existence of the positive electrode in the neighborhood of the plasma, the secondary electron in the neighborhood of the positive electrode is absorbed by the positive electrode of different polarity. The quantity of the secondary electron existing in the plasma is reduced as well accordingly. Incidentally, since the primary electron produced from the electron producing source has a comparatively high directivity and is trapped by the magnetic field to move along the magnetic field, the rate of the primary electron absorbed by the positive electrode is far smaller than the secondary electron. In order to further reduce the rate of the primary electron absorbed by the positive electrode, it is preferred to more intensify the magnetic field produced by the magnetic field generator so as to cause the magnetic field to intensively trap the primary electron.
Since the secondary electron has the comparatively small energy as above described, it scarcely contributes to the production of the multiply charged ion, but contributes only to the production of the singly charged ion. Since the quantity of the secondary electron is reduced owing to the existence of the positive electrode, the singly charged ion produced in the plasma will be reduced correspondingly. Viewing it differently, the rate of the multiply charged ion in the plasma is relatively increased.
With the actions of the preceding (1) and (2), the rate of the multiply charged ion in the plasma may be increased, and in turn the rate of the multiply charged ion contained in the ion beam may be increased. As a result, the quantity of the multiply charged ion to be extracted may be increased without totally increasing the ion beam current (ion beam extraction quantity).